The role of kinetic instabilities in formation of the runaway electron current after argon injection in DIII-D

Kinetic instabilities in the MHz range driven by runaway electrons (REs) have been observed for the first time during the current quench (CQ) in disruptions triggered by massive injection of argon in DIII-D. These instabilities are well-correlated with intermittent RE losses in the beginning of RE current formation. The runaway current phase is not observed when the power of instabilities exceeds a threshold. Novel measurements of the RE distribution function during the CQ indicate that the instabilities appear when RE energy (E _RE) exceeds 2.5-3 MeV, the number of modes grows linearly with E _RE, and their frequencies lie in the range 0.1-3 MHz, below the ion cyclotron frequency. Possible plasma waves exciting by REs in this region are proposed. Increase of the amount of injected argon decreases the E _RE and increases the success rate of the runaway current formation, while increase of the pre-disruption plasma current acts in the opposite direction. No dependence on the pre-disruption core electron temperature is found.